In the bleaching departments of chemical pulp mills, the fibres are normally transported, in the form of a fibre pulp suspension, from a maintenance vessel, for example a so-called standpipe, using a pump which is directly connected to the maintenance vessel. This pump generates a pressure which is sufficiently high to enable the suspension to be transported to the reactor vessel and introduced through the bottom of the latter. On the way, aqueous steam is supplied in a mixer in order to heat the suspension from the said temperature which does not exceed 95.degree. C., for example room temperature or normally approximately 70.degree. C., to a temperature which is suited to the reaction and which is at least 100.degree. C., normally greater. In order to achieve this, the pump pressure has to overcome the losses in pressure along the transportation distance, including a possible loss in pressure through the mixer, and, in particular, overcome the counterpressure in the bottom part of the reactor. This pressure can be considerable, since the reactors in question are tall, giving rise to a high static pressure at the bottom, and, in addition, a process pressure is applied to them. This means, in turn, that it is necessary, to achieve the heating, to use steam which is under a relatively high pressure, usually so-called medium-pressure steam which is available in the pulp mill and which is normally at a pressure of 9-12 bar. A disadvantage of this technique is that medium-pressure steam is expensive. Another limitation is the fact that it becomes difficult, or is rendered impossible, to use a mixer of the type which results in a pressure fall. This is a drawback, since these types of mixer possess certain advantageous properties, such as being relatively simple and cheap.